The present invention is directed to connecting members for coupling frame members of a screen printing machine. In particular, the present invention is directed to a connecting member which pre-cambers the frame member prior to mounting the screen.
Screen printing machines utilize a tensioned screen of fabric or mesh mounted on a supporting frame. A squeegee forces ink or other fluid media through the screen over an area whose shape is defined by a stencil. The squeegee is moved under pressure across the screen to deflect the screen downwardly toward the substrate to provide for transfer of the fluid medium onto the substrate.
The screen of a screen printing machine is typically secured to a rectangular frame having coupled members which retain the edges of the tensioned screen. Prior art frames include frames known as xe2x80x9cstretch and gluexe2x80x9d frames in which a screen is adhered to a supporting frame while in a tensioned condition. Also known in the art are frames known as xe2x80x9croller framesxe2x80x9d in which rollers are coupled together and rotatably supported by connecting members. Each of the rollers retains an edge of the screen for tensioning of the screen through rotation of the rollers. A locking mechanism secures the rollers to the connecting members to maintain the desired tension in a print screen. A typical roller of a roller frame includes a hollow cylindrical tube made of aluminum and having a longitudinally extending channel in which an edge of the screen is retained.
For larger frames, the coupled members of the frame can become long enough that the members may become subject to undesirable transverse deflections. Such deflections may include inwardly directed deflections resulting from loads applied to the frame by the tensioned screen. As a result of the inward deflections, the printing area of the screen may become distorted leading to loss of print area and undesirable inconsistencies during printing.
Reinforcement structures for frames to resist deflections of frame members are known in the art. For example, U.S. Pat. No. 3,908,293 to Newman, which is incorporated herein by reference, discloses a roller frame in which the rollers supporting the screen are coupled together by corner members. The roller frame includes a tensioning member extending between the corner members to camber the rollers. The tensioning members are located inwardly from the rollers at a distance from the rollers and therefore encroach towards the print area of the screen. The ""293 patent teaches that the corner members are flexible so that the legs of the corner members can rotate in order to achieve the desired outward cambering of the roller. However, the necessary flexibility of the corner members also leads to warping of the frame from undesirable deflections of the corner legs out of the plane of the frame.
U.S. Pat. No. 4,345,390 to Newman, which is also incorporated herein by reference, shows a roller frame having an inwardly disposed roller support member associated with each roller. The support member is located adjacent the roller to resist inward deflection of the roller resulting for example from loading applied to the roller by a tensioned screen. The disclosed support structures are passive structures which have not been cambered prior to tensioning of a supported screen.
What is needed is a rigid connecting member for coupling elongated frame members to form a frame of a screen printing machine in which an angular misalignment between connecting surfaces of connecting members and the frame member causes a pre-cambering of the frame members during the process of coupling the frame members to form the frame. The provision of pre-cambering by the connecting members eliminates the need for additional reinforcing members. This represents weight savings and size reduction over prior art frames directed to cambering which require additional frame reinforcing members.
According to the present invention there is provided a frame for supporting the screen of a screen printing machine. The frame includes elongated frame members which retain the edges of a tensioned screen. The frame also includes rigid connectors coupling the frame members together. The connectors have a pair of edges defining connecting surfaces for connection to end surfaces of the frame members also defining connecting surfaces. Either the connecting surfaces of the connectors or the connecting surfaces of the frame members are adapted to create an angular misalignment between the connecting surfaces of the elongated frame members and rigid connectors during the coupling of the frame members. This angular misalignment forces a pre-cambering of the frame members resulting in substantial alignment between the connecting surfaces.